A 20th birthday episode of the show aired on Tuesday evening in the US, with William among a number of special faces making an appearance.
A pre-recorded clip showed Irwin, son of the late wildlife conservationist Steve Irwin, and his dance partner Witney Carson receiving a Zoom call from the future king in the lead-up to their performance.
"We're missing you Robert," William said on the call, which took place ahead of the Earthshot awards night in Rio de Janeiro last week.
Irwin, 21, had to miss the ceremony due to his commitments with the US version of Strictly Come Dancing.
"Whilst your 'twinkle toes' are going off elsewhere, I need you down here," William said.
"Witney, you need to get him in as much glitter as you can. You guys have got a seriously good chance of winning it. So just the best of luck on the show."
Irwin and Carson went on to receive a perfect score for their Foxtrot performance, which Irwin dedicated to his father.
The prince used last week's Earthshot awards to hail the next generation of environmentalists.
The annual event, now in its fifth year, aims to recognise and scale up solutions to "repair" the planet, with five winners awarded one million pounds ($A2 million) each to develop their ideas.
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